String, byte array

In Swift 5.9 a byte is called a UInt8—an unsigned 8 bit integer. A byte array is a UInt8 array. In ASCII we can treat chars as UInt8 values.

With the utf8 String property, we get a UTF8View collection. We can convert this to a byte array. Then we can manipulate bytes. We can convert those bytes into a String.

Example

This example takes a String with value "PERLS" and gets its UTF8View. Then it creates a byte array from the UTF8View with an array creation statement.

import Foundation

// An input string.
let name = "PERLS"

// Get the String.UTF8View.
let bytes = name.utf8
print(bytes)

// Get an array from the UTF8View.
// ... This is a byte array of character data.
var buffer = [UInt8](bytes)

// Change the first byte in the byte array.
// ... The byte array is mutable.
buffer[0] = buffer[0] + UInt8(1)
print(buffer)

// Get a string from the byte array.
if let result = String(bytes: buffer, encoding: String.Encoding.ascii) {
    print(result)
}
PERLS
[81, 69, 82, 76, 83]
QERLS

Create string, bytes

It is possible to create a String from a sequence of bytes. We can specify these directly in the program. This allows dynamic creation of strings.

import Foundation

// An empty UInt8 byte array.
var buffer = [UInt8]()

// Add some bytes to the byte array.
buffer.append(UInt8(97))
buffer.append(UInt8(98))
buffer.append(UInt8(99))

// Get a string from the byte array.
if let result = String(bytes: buffer, encoding: String.Encoding.ascii) {
    print(result)
}
abc

In Swift, Strings are hard to manipulate. Using a UInt8 array to create certain kinds of strings (like those meant for machines) is a good strategy to improve performance and gain clarity.

Some notes

In Swift we have access to a utf8 property. This is key to converting Strings to byte arrays. We use the UInt8 type to indicate a byte in this language.